1. Field of the Invention
The invention relates generally to computer displays and, more particularly, to a method for the optimum selection of a small number of colors from a large color palette for color imaging.
2. Description of the Prior Art
The use of photographic quality color images for display on a cathode ray tube device has become increasingly important for many computer applications, including those implemented for small personal computers. Most of the less expensive display devices, however, can only display a very limited number of colors at one time. This is primarily due to economics. For example, many displays assign as few as four bits per picture element (pel) which limits them to presenting no more than sixteen colors at a time. These colors are referred to as the presentation colors and are usually selected from a larger color palette. The number of distinct colors available in a color palette from which the sixteen presentation colors may be chosen can be made relatively large for a relatively small cost. This is done simply by implementing sixteen writable register sets that are selectable during each refresh cycle of the display. The color number from each pel of the image is used as a register set selector. The values from the selected register set are then routed to produce the red, green and blue CRT voltages as needed.
As the power of small computers increases, the power of imaging techniques has likewise been increased. Thus the demands for the color images displayed on a display CRT device associated with such computers has increased as well. As imaging requirements have increased so has the need to produce photographic quality images on the computer display. Heretofore, it has been difficult to provide a photographic quality image on a computer display where the number of presentation colors is limited to as few as sixteen.
The human eye, with a normal pathology, is capable of distinguishing approximately 350,000 different colors. This number has been experimentally established by direct comparison of pairs of colors placed side by side. In such experiments the viewer is asked to state whether the two colors are the same or different. Altogether, approximately 128 hues are distinguishable. Except for the spectral extreme, the wavelength of the distinguishable hues lie within three nanometers of those of their spectral neighbors. If the colors vary only in saturation, the eye can distinguish from 16 (for yellow) to 23 (for red and violet) intensity levels. All of these measurements are made using natural sunlight as illumination.
The three phosphors of color CRT's generally are not capable of producing all the hues and saturations available in natural sunlight. Thus, an approximation of the human visual limit can be produced employing no more than six bits of resolution per primary, which yield 64 discrete intensity levels per primary or 262,144 (64.sup.3) distinct colors.
Clearly, in order to display a satisfactory color image on the CRT device attached to a computer, it has been necessary to compromise on the selection of which presentation colors to use to provide the most natural looking representation of, for example, a naturally occurring visual scene. It is possible to produce a natural looking image if the few presentation colors are carefully selected for a specific image.
Prior art techniques for accomplishing the digitization of a color have included a method described in "Color Image Quantization for Frame Buffer Display," P. Heckbert, Computer Graphics, Vol. 16, No. 3, pp. 297-307, July 1982. In the method described by Heckbert, the original image is (1) sampled for color statistics, (2) a color map is chosen based on the statistics, (3) the original colors are mapped to their nearest neighbors in the color map and (4) the original image is redrawn.